The ANSS event ID is ak0191pitgo3 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0191pitgo3/executive.
2019/02/06 20:04:45 61.408 -150.030 34.3 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/02/06 20:04:45:0 61.41 -150.03 34.3 4.1 Alaska
Stations used:
AK.CUT AK.FIRE AK.GHO AK.HIN AK.KLU AK.KNK AK.PWL AK.RC01
AK.SAW AK.SCM AK.SKN AK.SLK AK.SSN AK.SWD AT.PMR AV.ILSW
AV.STLK TA.M22K TA.O22K
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 2.51e+22 dyne-cm
Mw = 4.20
Z = 53 km
Plane Strike Dip Rake
NP1 210 80 -80
NP2 345 14 -135
Principal Axes:
Axis Value Plunge Azimuth
T 2.51e+22 34 291
N 0.00e+00 10 28
P -2.51e+22 54 132
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.60e+21
Mxy -1.52e+21
Mxz 1.23e+22
Myy 1.01e+22
Myz -1.98e+22
Mzz -8.46e+21
##########----
##################---#
######################--####
#####################------###
######################---------###
######################-----------###
#####################--------------###
###### ############----------------###
###### T ###########-----------------###
####### ##########-------------------###
###################--------------------###
##################---------------------###
#################----------------------###
###############---------- ----------##
##############----------- P ---------###
############------------ ---------##
##########------------------------##
########------------------------##
######-----------------------#
####----------------------##
#--------------------#
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Global CMT Convention Moment Tensor:
R T P
-8.46e+21 1.23e+22 1.98e+22
1.23e+22 -1.60e+21 1.52e+21
1.98e+22 1.52e+21 1.01e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190206200445/index.html
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STK = 210
DIP = 80
RAKE = -80
MW = 4.20
HS = 53.0
The NDK file is 20190206200445.ndk The waveform inversion is preferred.
Given the availability of digital waveforms for determination of the moment tensor, this section documents the added processing leading to mLg, if appropriate to the region, and ML by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for ML is adjusted to remove a linear distance trend in residuals to give a regionally defined ML. The defined ML uses horizontal component recordings, but the same procedure is applied to the vertical components since there may be some interest in vertical component ground motions. Residual plots versus distance may indicate interesting features of ground motion scaling in some distance ranges. A residual plot of the regionalized magnitude is given as a function of distance and azimuth, since data sets may transcend different wave propagation provinces.
Left: ML computed using the IASPEI formula for Horizontal components. Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
Left: ML computed using the IASPEI formula for Vertical components (research). Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
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The focal mechanism was determined using broadband seismic waveforms. The location of the event (star) and the stations used for (red) the waveform inversion are shown in the next figure.
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The program wvfgrd96 was used with good traces observed at short distance to determine the focal mechanism, depth and seismic moment. This technique requires a high quality signal and well determined velocity model for the Green's functions. To the extent that these are the quality data, this type of mechanism should be preferred over the radiation pattern technique which requires the separate step of defining the pressure and tension quadrants and the correct strike.
The observed and predicted traces are filtered using the following gsac commands:
cut o DIST/3.3 -30 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 300 85 -10 3.21 0.1624
WVFGRD96 2.0 120 75 -5 3.37 0.2244
WVFGRD96 3.0 120 70 -10 3.45 0.2580
WVFGRD96 4.0 120 65 -10 3.51 0.2786
WVFGRD96 5.0 120 65 -10 3.54 0.2929
WVFGRD96 6.0 120 65 -15 3.58 0.3092
WVFGRD96 7.0 120 65 -15 3.61 0.3239
WVFGRD96 8.0 120 65 -20 3.66 0.3346
WVFGRD96 9.0 120 65 -20 3.68 0.3393
WVFGRD96 10.0 120 65 -15 3.70 0.3399
WVFGRD96 11.0 120 65 -15 3.71 0.3390
WVFGRD96 12.0 120 65 -15 3.73 0.3369
WVFGRD96 13.0 125 70 -15 3.74 0.3367
WVFGRD96 14.0 125 65 -15 3.75 0.3384
WVFGRD96 15.0 125 65 -15 3.76 0.3412
WVFGRD96 16.0 125 65 -15 3.78 0.3456
WVFGRD96 17.0 225 70 10 3.79 0.3491
WVFGRD96 18.0 225 70 10 3.81 0.3543
WVFGRD96 19.0 225 70 10 3.82 0.3584
WVFGRD96 20.0 225 70 10 3.83 0.3635
WVFGRD96 21.0 225 70 10 3.84 0.3681
WVFGRD96 22.0 225 70 10 3.85 0.3736
WVFGRD96 23.0 225 65 15 3.86 0.3796
WVFGRD96 24.0 225 65 15 3.88 0.3860
WVFGRD96 25.0 225 65 15 3.88 0.3916
WVFGRD96 26.0 225 65 15 3.89 0.3974
WVFGRD96 27.0 225 65 15 3.90 0.4021
WVFGRD96 28.0 225 65 15 3.91 0.4063
WVFGRD96 29.0 225 65 15 3.92 0.4093
WVFGRD96 30.0 90 35 -15 3.93 0.4095
WVFGRD96 31.0 85 35 -20 3.94 0.4184
WVFGRD96 32.0 80 30 -30 3.96 0.4296
WVFGRD96 33.0 75 20 -35 3.97 0.4462
WVFGRD96 34.0 75 20 -35 3.98 0.4628
WVFGRD96 35.0 210 85 -80 4.00 0.4835
WVFGRD96 36.0 205 80 -80 4.00 0.5019
WVFGRD96 37.0 210 80 -80 4.01 0.5190
WVFGRD96 38.0 210 80 -80 4.01 0.5340
WVFGRD96 39.0 210 80 -75 4.01 0.5461
WVFGRD96 40.0 210 85 -80 4.16 0.5486
WVFGRD96 41.0 210 80 -80 4.16 0.5536
WVFGRD96 42.0 210 80 -80 4.16 0.5574
WVFGRD96 43.0 210 80 -80 4.17 0.5606
WVFGRD96 44.0 210 80 -80 4.17 0.5619
WVFGRD96 45.0 210 80 -80 4.17 0.5650
WVFGRD96 46.0 210 80 -80 4.18 0.5653
WVFGRD96 47.0 210 80 -80 4.18 0.5683
WVFGRD96 48.0 210 80 -80 4.19 0.5695
WVFGRD96 49.0 210 80 -80 4.19 0.5709
WVFGRD96 50.0 210 80 -80 4.19 0.5722
WVFGRD96 51.0 210 80 -80 4.20 0.5721
WVFGRD96 52.0 210 80 -80 4.20 0.5724
WVFGRD96 53.0 210 80 -80 4.20 0.5726
WVFGRD96 54.0 210 80 -80 4.21 0.5704
WVFGRD96 55.0 210 80 -80 4.21 0.5703
WVFGRD96 56.0 210 80 -80 4.21 0.5686
WVFGRD96 57.0 205 80 -80 4.22 0.5663
WVFGRD96 58.0 205 80 -80 4.22 0.5642
WVFGRD96 59.0 205 80 -75 4.23 0.5624
The best solution is
WVFGRD96 53.0 210 80 -80 4.20 0.5726
The mechanism corresponding to the best fit is
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The best fit as a function of depth is given in the following figure:
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The comparison of the observed and predicted waveforms is given in the next figure. The red traces are the observed and the blue are the predicted. Each observed-predicted component is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. A pair of numbers is given in black at the right of each predicted traces. The upper number it the time shift required for maximum correlation between the observed and predicted traces. This time shift is required because the synthetics are not computed at exactly the same distance as the observed, the velocity model used in the predictions may not be perfect and the epicentral parameters may be be off. A positive time shift indicates that the prediction is too fast and should be delayed to match the observed trace (shift to the right in this figure). A negative value indicates that the prediction is too slow. The lower number gives the percentage of variance reduction to characterize the individual goodness of fit (100% indicates a perfect fit).
The bandpass filter used in the processing and for the display was
cut o DIST/3.3 -30 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3
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| Figure 3. Waveform comparison for selected depth. Red: observed; Blue - predicted. The time shift with respect to the model prediction is indicated. The percent of fit is also indicated. The time scale is relative to the first trace sample. |
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| Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the waveforms. Each solution is plotted as a vector at a given value of strike and dip with the angle of the vector representing the rake angle, measured, with respect to the upward vertical (N) in the figure. |
A check on the assumed source location is possible by looking at the time shifts between the observed and predicted traces. The time shifts for waveform matching arise for several reasons:
Time_shift = A + B cos Azimuth + C Sin Azimuth
The time shifts for this inversion lead to the next figure:
The derived shift in origin time and epicentral coordinates are given at the bottom of the figure.
The WUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows (The format is in the model96 format of Computer Programs in Seismology).
MODEL.01
Model after 8 iterations
ISOTROPIC
KGS
FLAT EARTH
1-D
CONSTANT VELOCITY
LINE08
LINE09
LINE10
LINE11
H(KM) VP(KM/S) VS(KM/S) RHO(GM/CC) QP QS ETAP ETAS FREFP FREFS
1.9000 3.4065 2.0089 2.2150 0.302E-02 0.679E-02 0.00 0.00 1.00 1.00
6.1000 5.5445 3.2953 2.6089 0.349E-02 0.784E-02 0.00 0.00 1.00 1.00
13.0000 6.2708 3.7396 2.7812 0.212E-02 0.476E-02 0.00 0.00 1.00 1.00
19.0000 6.4075 3.7680 2.8223 0.111E-02 0.249E-02 0.00 0.00 1.00 1.00
0.0000 7.9000 4.6200 3.2760 0.164E-10 0.370E-10 0.00 0.00 1.00 1.00